The Brimar CV2129 is commercially designated as the 5763 by the maker. It is a small transmitting beam tetrode valve for HF and low VHF use and is equivalent to the Mullard QV03-12.
The circular anode contains a flat sided cathode and grids that are flat across the cathode width and rounded at the ends. The screen grid wires are directly in front of each control grid wire.
The heater is referenced as 6.0 V by RCA, Brimar and Mullard.
John Fielding ZS5JF has sent the following information and the RCA datasheet.
The 5763/CV2129 was extensively used as an oscillator, multiplier or power amplifier in HF and VHF equipment. The old Marconi Instruments Q meter variable frequency oscillators used it, also used in the PSU as a series regulator valve. The useful upper frequency limit is about 300 MHz. It was also used as a driver or PA in some early Pye Telecomms VHF mobile radios before being superseded by the twin tetrode QQV03-10 type of valves.
The anode face view.
The grids can be seen to pass flat across the cathode. The beam plates can be seen just inside the anode notch.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 58 mm tall.